Arsalaan N. Pathan

Catalytic processes developed for petroleum refining, such as bifunctional hydrocracking, can be adopted to chemically recycle plastic polymers. In these reactions, polymer substrates often exhibit distinct kinetics from small molecules with identical chemical structures, emphasizing that the macromolecular behavior of polymer chains beyond local bonding modulates the reactivity. Here, we report the direct measurement of the dynamics of polymer chains at a model catalytic interface, using quasielastic neutron scattering (QENS). Three polypropylenes (PPs) of varying chain lengths and tacticities were studied. In all cases, polymer–catalyst interaction leads to the loss of bulk melting behavior, and segmental dynamics of PP chains can be described by continuous translational diffusion. A lower activation barrier and less favored entropy change at the transition state were observed at the catalytic interface relative to bulk melts, leading to reduced segmental diffusivity at reaction temperatures. The interfacial dynamics are faster for isotactic PP than for atactic PP. This difference is absent in the melt and rationalizes the higher reactivity of isotactic PP in diffusion-limited hydrocracking. This work pioneers the use of H-sensitive neutron-based techniques to probe the consequential macromolecular behaviors of reacting polymer chains at catalytic interfaces. The integration of such novel fundamental knowledge into bonding-level chemical kinetics can transform the understanding of catalytic polymer recycling reactions into a holistic framework, enabling the accurate explanation and prediction of phenomena not accounted for by traditional kinetic approaches.

A self-cross-linking polyisobutylene thiol (PIB thiol) was developed in this work from brominated isobutylene-co-para methylstyrene (BIMSM rubber). Nucleophilic substitution of BIMSM benzylic bromides using sulfur-based nucleophiles yields a shelf-stable intermediate PIB xanthate, which can be readily cleaved to produce thiol-grafted PIB. The PIB thiols oxidatively dimerize at elevated temperatures to yield a cross-linked network, reinforced solely through disulfide linkages. In the absence of accelerators, elemental sulfur, and other additives, this “traceless vulcanization” affords networks with up to 99% gel fractions and mechanical properties that can reach up to 1000 kPa in tensile strength, 1200% elongation, and shape recovery. Rheological analysis and cross-link density characterization provided insights into the reinforcement nature of a purely disulfide network in butyl rubbers with high creep resistance, along with low tan(δ) values across a wide range of temperatures and test frequencies.